1. Field of the Invention
This invention relates to a head position controlling device advantageously employed in, for example, a magneto-optical disk apparatus.
2. Related Art
In magneto-optical disc apparatus of a magnetic field modulation system in which modulation signals based on information signals to be recorded on a magneto-optical disc are applied on a magnetic field generating device adapted for generating an external magnetic field, or of an optical modulation system in which the aforementioned modulation signals are applied on a laser light source side, there is provided a controlling device whereby, for assuring satisfactory writing and erasure of information signals on or from the magneto-optical disc, the relative distance between a magnetic field generating coil (magnetic head) and a magneto-optical disc is maintained at a constant value. This type of the controlling device has been proposed by the present assignee in, for example, the JP Utility Model Application No.1-46200 (1989).
This controlling device is shown in FIG. 1 wherein a gap sensor including a light-emitting element 54 for irradiating a surface 51a of a magneto-optical disc 51 facing a magnetic field generating coil 52 with a diffused light and a pair of light-receiving elements 55, 56 for receiving the diffused light from the light-emitting element 54 reflected from the facing surface 51a and for generating an output corresponding to the volume of the received light is embedded in a bobbin 53 in which there is also embedded the magnetic field generating coil 52 for applying the magnetic field on the magneto-optical disc 51.
A differential amplifier 57 for detecting the level difference of output voltages of the light-receiving elements 55, 56 is connected to these light-receiving elements 55, 56, and a control current corresponding to an output voltage of the differential amplifier 57 is applied via a driver circuit 58 to a driving coil 59 provided about the outer periphery of the bobbin 53, so that the magnetic field generating coil 52 is driven in a direction towards and away from the magneto-optical disc 51, as shown by an arrow T in FIG. 1.
That is, with the above described controlling apparatus, a relative distance d between the magnetic field generating coil 52 and the magneto-optical disc 51 is adapted to be maintained constant by performing a servo control operation so that, with the relative distance between the coil 52 and the disc 51 when the output voltages from the light-receiving elements 55, 56, is reduced to zero, that is, the relative distance between the coil 52 and the disc 51 when the level difference between these light-receiving elements 55 and 56 is reduced to zero, as a reference, the level difference between the output voltages from the light-receiving elements 55 and 56 will always be equal to zero.
However, it may occur that,if the light-emitting element 54 and the light-receiving elements 55 and 56 are arrayed along the radius of the magneto-optical disc 51 , the servo control operation cannot be applied. For example, if the light-emitting element 56 is positioned in opposition to a ROM (PROM) region having a pre-formed pit row(s) and the remaining light-emitting element 55 is positioned in opposition to a RAM region (a data-recording region), the reflectance in the ROM region is drastically lowered as compared with that in the RAM region, so that, even though the relative distance d remains the same, the level difference of the output voltages from the light-receiving elements 55, 56 is increased suddenly to shift the zero-crossing points of the output voltages to render it impossible to apply a servo for reducing the level difference to zero.
For combatting such drawback, the present assignee has previously proposed an arrangement in which a gain-modifying amplifier is connected between the light-receiving element 56 and the differential amplifier 57 exhibiting a lower output voltage level and the output voltage level from the light-receiving element 56 is raised for correcting the equilibrium of the output voltages from the light-emitting elements 55 and 56 to enable a servo to be applied for reducing the output voltage level difference to zero.
However, with the proposed method, circuit construction is complicated due to provision of the amplifier so that inexpensive devices cannot be provided.